refactor: minor refactor for table header background

[pranabdas]

Changes made:

• Distinct background color for table header (see https://github.com/facebookincubator/infima/pull/213#issuecomment-1072230336)
• Rows are highlighted on hover. I think helpful to follow along wide table rows with many columns.
• Reduce vertical padding for table rows. I think the table rows are too relaxed vertically.

As suggested, the earlier changes that I proposed should be done at the user land. I reverted the changes, only a minor refactor there for the table header color.

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